The two main complications of deep vein thrombosis (DVT) are pulmonary embolism and post-thrombotic syndrome.

Pulmonary embolism

A pulmonary embolism is the most serious complication of DVT. It happens when a piece of blood clot (DVT) breaks off and travels through your bloodstream to your lungs, where it blocks one of the blood vessels. In severe cases this can be fatal.

About one in 10 people with an untreated DVT develops a severe pulmonary embolism.

Post-thrombotic syndrome

If you've had a DVT, you may develop long-term symptoms in your calf known as post-thrombotic syndrome. This affects around 20-40% of people with a history of DVT.

If you have DVT, the blood clot in the vein of your calf can divert the flow of blood to other veins, causing an increase in pressure. This can affect the tissues of your calf and lead to symptoms, including:

When a DVT develops in your thigh vein, there's an increased risk of post-thrombotic syndrome occurring. It's also more likely to occur if you're overweight or if you've had more than one DVT in the same leg.